In my Semi-Serious headcanon: The Foundation gives then a specialized amnesiac so they forget the whole month, and next month they repeat ad infinitum. Of course, some scips require blood, so some of the less stable ones "disappear" while the Class-D amnesiacs are kicking in.

In my Reallulcanon: Staff go to their offices and find a box. On the box is a note with messily scribled text, in crayon:

JUST DO WHAT COMES NATURAL

Upon opening the box, researchers will find a random firearm and a Site map with the D-Class dorms scribbled over with highlighter.

1) The kind of SCPs they are exposed to
2) How well they perform
3) Evaluation of behavior and mental health

D-class working with some Safe subjects will probably be given a prolonging (some days or maybe another month at most) without memory wipe or termination. These people are probably an incredible minority to reduce the risk of prison breaks, organization of riots or using SCPs to their advantage. Memory wipes would be done to D-class members that are either being moved to another project or are carrying too much knowledge about Foundation members or a particular SCP.

When it comes to Keter and maybe the more dangerous Euclid subjects, termination would be made mandatory in case the amnesiacs fail to remove some kind of mental or behavioral influence done upon the D-class member.

My headcanon? They die. Not because they're terminated, but just because their organs have failed.

I subscribe to the theory (as I wrote out in Walk the Floor) that D-class (and other classes of personnel) are bred en mass via 597. D-Class are meant to be disposable: they're cheaply made, designed to work for a month off of nutrient paste, and then die, and the biomass is reprocessed.

I think that they are sent back to Death row. But at the front of the line. Actually, probably about ten feet in front of the line. Probably just skip the line metaphor all together, and just put them in the room, after adjusting their vocal chords so they can't say anything.
Or they just pull a "give them 5 minutes to run, then send the dogs killer robots."